Hot water heater

ABSTRACT

A hot water heater has an inlet for liquid fuels, a plurality of inlets forresh air, an inlet for a fluid to be heated, at least two combustion stages traversed by the fuel-air mixture with catalytic combustion chambers surrounded at least partially by at least one fluid chamber filled with fluid and with an offgas heat exchanger for fluid to be heated. The heat exchanger is traversed by the offgas escaping from the combustion chambers. The first combustion stage has an evaporation chamber that has on its outside surface, at least a partial catalyst layer.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a hot water heater with an inlet for liquidfuels, a plurality of inlets for fresh air, an inlet for a fluid to beheated, at least two combustion stages which are traversed by thefuel-air mixture and have catalytic combustion chambers surrounded atleast partially by at least one fluid chamber filled with fluid, andwith an offgas heat exchanger for the fluid to be heated, which istraversed by the offgas escaping from the combustion chambers.

A hot water heater as generally described above is known for examplefrom German Patent document DE-OS 33 32 572. In addition, a hot waterheater is likewise described in German Patent document 42 04 320.4wherein the heater, in particular has a first advantageous combustionstage. This reference provides background information on the first andsecond combustion stages. For improved clarity, the reference numeralsin this application partially correspond to those in German Patentdocument 42 04 3204.

When fossil fuels are burned, in addition to the greenhouse gas carbondioxide, additional pollutants such as sulfur dioxide and oxides ofnitrogen are produced. In conventional flame burners, the reductionpossibilities, primarily for the oxides of nitrogen, are limited byflame stability and the formation of carbon monoxide. A definitereduction in the emission of oxides of nitrogen can be achieved inflameless combustion on oxidation catalysts (platinum for example) as aresult of the low reaction temperature. Catalytic burners also offer theadvantage that mixtures of fuels with different energy densities can bereacted stably over a wide range of mixing ratios.

Burners for gasoline, diesel fuel, or methanol for example, areavailable today only as conventional flame burners. Because of the highreaction temperature (flame temperature) such burners have high nitrogenoxide emissions. There are ways in which emissions can be reduced evenin such burners, for example by flame cooling or changing the percentageof air, but this causes the flame stability to decrease and the carbonmonoxide emissions to increase.

This prior art has the disadvantage that it is not especially suitablefor liquid fuels. Hence, there is therefore needed an improved hot waterheater such that liquid fuels can be used without significant cracking.

The present invention meets this need by providing a hot water heaterwith an inlet for liquid fuels, a plurality of inlets for fresh air, aninlet for a fluid to be heated, at least two combustion stages which aretraversed by the fuel-air mixture and have catalytic combustion chamberssurrounded at least partially by at least one fluid chamber filled withfluid, and with an offgas heat exchanger for the fluid to be heated,which is traversed by the offgas escaping from the combustion chambers.The first combustion stage has an evaporation chamber which has at leastpartially on its outer surface, a catalyst layer of a catalyticcombustion chamber of the first combustion stage. The invention makes itpossible to thermally couple the first stage of the two-stage catalyticburner to the evaporation chamber.

One advantageous improvement on the catalytic cracking burner is thatthe first catalytic combustion chamber of the first combustion stage isdesigned as a catalytic cracking burner.

It is also advantageous for the evaporation chamber to be designed as acombustion chamber, for which purpose it has an ignition device. Abypass for the feed for the liquid fuel can serve as the igniting flamefor example. In addition, the hot water heater has a supply of primaryair for the combustion chamber for this purpose.

It is further advantageous if the fuel is supplied in isolation so thatthe fuel enters the combustion chamber without cracking.

It is also advantageous to provide a nozzle or other devices foratomizing the fuel.

It can also be advantageous to recycle a portion of the offgas from thefirst stage into the evaporation chamber, since the liquid fuel is thenevaporated more easily and the water or steam that is created during thecombustion process likewise minimizes any possible cracking reactions.

It is further advantageous to provide devices for directing the gasstream inside the combustion chamber; in particular these devices can bethermally insulated if they are not heated by the first combustionstage.

It is also advantageous to make the evaporation chamber rotationallysymmetrical and to cause it to rotate, since the fuel is then pressedagainst the wall and comes into better contact there with the wall thatis heated on the back by the first combustion stage because of theconversion reaction of the combustion gas-air mixture on the catalystlayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional representation of a catalytic burneraccording to the present invention;

FIG. 2 is a cross-sectional representation of the catalytic burner inFIG. 1 with openings provided from the gas chamber between the first andsecond stages; and

FIG. 3 is a cross-section representation of a catalytic burner in whicha highly porous structure is provided inside the catalyst tube.

DETAILED DESCRIPTION OF THE DRAWINGS

The subject of the application is a two-stage catalytic burner forliquid fuels and their mixtures with internal evaporation orgasification. The fuel, possibly with the addition of air (primary air)is evaporated or gasified inside the burner. The energy required forthis purpose is provided by the heat of combustion. The mixture ofcombustion gas and air (with secondary air added, which can be the solesupply of air after the starting phase) flows over a catalytic surfaceand is converted up to approximately 80 to 85%. The reactiontemperatures are approximately 800° to 900°C. Heat is drawn off into thecooling medium and the evaporation zone by radiation, heat conduction,and convection. In the second stage the remaining fuel is reacted in amonolithic catalyst. The narrow channels ensure good material transportand hence high power density. Temperatures of approximately 1,000° C.are reached and permit a complete reaction. A portion of the heat can bedrawn off from the monolith for preheating the primary air, which isadvantageous, for example, with intermittent operation.

Referring to FIG. 1, the catalytic burner consists of two stages 16, 20.The first stage 16 consists of a metal tube 31 coated on the outsidewith the catalyst 13, such as a ceramic tube. This catalyst tube 13 issurrounded by a ceramic or metal tube 11 which functions as an exhaustgas heat exchanger, and a cooling jacket 12 having a fluid chamber 4 fora cooling medium 2, so that a gas gap 15 results between the catalysttube 13 and the ceramic tube 11. The mixture of evaporated gaseous fueland air flows in this gas gap 15 and reacts at the catalytic surface oftube 31. The second stage 20, located above the first stage 16, consistsof a ceramic honeycomb structure 17 (monolith) coated with catalyst. Theoffgas (exhaust gas) from the first stage 16 together with the remainingfuel flows through this monolith 17 where it reacts completely. Thesource of the primary air and liquid fuel mixture is located centrallyin the monolith.

Two tubes 8 and 41 arranged concentrically and passing through themonolith 17 from above serve to introduce the liquid fuel and primaryair. The primary air flows in outer tube 41 and is preheated by theadjoining monolith 17. The liquid fuel flows in the inner tube 8. Theinner tube 8 is only slightly preheated since the gas gap between thistube and the monolith has an insulating effect so that no evaporation orcracking can occur in the inner or "feed" tube 8. These concentric tubes8, 41 terminate at the level of the upper edge of the first burner stage16. The liquid fuel is added, finally atomized by means of a nozzle 42,to the interior of the catalyst tube 13 of the first burner stage 16which thus forms evaporation chamber 40 or the burner or combustionchamber. The concentrically supplied primary air, preheated by themonolith, likewise flows through holes provided in an annular fashionaround the fuel line 8 into the interior of the catalyst tube 13.Because a high percentage of primary air is provided, the liquid fuelcan be evaporated far below its boiling point. The addition of primaryair can be shut off after the starting phase in the favorable case. Inaddition, the fine atomization of the fuel produces a large evaporativesurface and the flow of the primary air produces good mass transferfigures. The energy for evaporation is provided by the heat of thepreheated air plus added heat (by conduction, convection, and radiation)from the catalyst tube 13.

The mixture of combustion gas and air flows downward inside theevaporation or burner chamber 40. A cone 45 placed on the bottom of theburner chamber 40 guides the gas at the lower end of the catalyst tube13 into the annular gap 15 between the ceramic tube, 11 and the catalysttube 13. At this point the secondary air 46 is added, entering directlyfrom below into the annular gas gap. The cone 45 has two primaryfunctions. It causes the mixture of combustion gas and air to flowuniformly into the annular gap 15. Without this cone 45, areas of deadspace could readily form at the bottom of the burner, at which anycracked products that might appear could collect. Another importantpoint is that the cone 45 is heated by the radiation from theevaporation chamber 40 (the back of the catalyst chamber). In this way,portions of the fuel can be prevented from condensing out again at thebottom of the burner or when they are guided into the gas gap.

The combustion gas/air mixture with secondary air added flows upward inthe annular gap 15 between the ceramic tube 11 and the catalyst tube 13.A portion of the fuel reacts with the catalytic surface. The energy thusreleased is distributed as follows: 1. the catalyst tube is heated orkept at the reaction temperature; 2. the reaction gas is heated; 3. heatis given-off or radiated to the interior of the catalyst tube; this heatis required to evaporate the liquid fuel mixture; the heat istransmitted by convection, conduction, and radiation; 4. heat islikewise given-off to the ceramic tube 11 from the catalyst tube throughconvection, conduction, and radiation; the heat is then given offfurther outward by conduction to the double jacket fluid chamber 4traversed by a cooling medium 2 (water or air) in the manner of anexhaust gas heat exchanger.

The catalyst tube has a temperature of approximately 700°-900° C.Approximately 80% of the fuel is reacted in this first stage.

From the annular gap 15 in the first stage 16, the gas mixture flowsupward into the expanded chamber 22 below the honeycomb 17 coated withcatalyst (platinum for example). The expansion of the cross sectionresults in a slowing of the flowrate and to additional thorough mixingahead of the second burner stage 20. The gas then flows through thenarrow channels in the catalyst honeycomb 17 where the remaining fuel iscompletely reacted. The good reaction yield in this second stage 20results from the following facts: 1. The mass transfer to the catalystis very good because of the narrow channels. 2. The low heat losses fromthe honeycomb and the production of heat by the reaction causes thehoneycomb to reach temperatures of about 900° to 1000° C.; the reactionrate at this temperature is so high that with the relatively longresidence time (low flowrate) the fuel can be reacted completely.Although only a very small amount of heat is carried away due to thepoor thermal conductivity of the ceramic monolith, the primary air,whose source is located at the center of the honeycomb, can be preheatedslightly. The preheating temperature of the primary air must not be toohigh in any case, since otherwise cracking reactions could occur when itencounters the atomized fuel.

The exhaust air from the second combustion stage is then utilized in aheat exchanger (not shown) to further heat the water or fluid 2 heatedin the first combustion stage.

The entire burner is started by lighting the flame in the evaporationchamber 40. The primary air flow is so great that complete combustion isensured. The flame heats the catalyst tube 13 from the inside byradiation, conduction, and convection. The hot offgases flow downward,are guided by the cone 45 at the bottom into the gas gap 5, and flowupward through the latter and through the honeycomb 17. The hot offgasgives up its heat, thus heating the burner with the honeycomb 17. Whenthe burner has reached a temperature at which the catalytic reaction canproceed at a suitably high rate (approximately 600° C.), the flame isshut off. This can be done by briefly turning off the primary air and/orthe fuel supply.

Any cracking products that appear and settle on the hot interior of thecatalyst tube 13 can be eliminated by lighting a flame at fixed timeintervals inside the catalyst tube. This flame is operated with excessair so that cracking products can be burned off the surfaces.

FIG. 2 shows the same burner once again, but in this case openings 44have been provided from the gas chamber 21 between the first 16 andsecond stages 20 to the interior of the catalyst tube (evaporationchamber) 40. These openings 44, which can be designed as nozzles, causea portion of the offgas from the first burner stage 16 to recirculatethrough the evaporation chamber 40. This offers the followingadvantages:

1. Lateral influx or intake of the offgas from the first stage ensuresthorough mixing and further dilution of the fuel/air mixture inside thecatalyst tube; this results in faster evaporation.

2. The hot offgas provides additional heat in the evaporation chamberthat is required for evaporation.

3. The steam that is present in the recirculated offgas and comes fromcombustion causes portions of the fuel to be reformed into carbonmonoxide or carbon dioxide and hydrogen, so that any cracking reactionsthat occur can be minimized.

4. With sufficient recirculation of the offgas, the primary air can beshut off.

FIG. 3 shows a burner in which a highly porous structure 43 is providedinside the catalyst tube 13. This structure 43 causes a portion of thefuel droplets that are sprayed into it, especially the larger ones, tobe deposited on the porous body and thus not come into contact with thehot wall of the catalyst tube. Because the temperatures are kept low atthis point, no cracking can occur. The porous structure 43 can consistof ceramic or metal and be designed as a parallelipiped, cylinder, ortube. The structure can also be coated with catalyst in order toaccelerate the evaporation reaction.

According to the invention, the first stage 16 of the two-stagecatalytic burner is thermally coupled to the evaporation chamber 40. Theevaporation chamber 40 simultaneously serves as the combustion chamberfor preheating. The thermal coupling between the first catalyzer stageand the evaporation chamber allows a flow of heat during the startingphase from this chamber, which is then the combustion chamber, into thefirst catalyzer stage and, during catalytic burner operation, converselyproduces a flow of heat from the first catalyzer stage to theevaporation chamber in order to produce the required evaporationenthalpy there.

The theoretical design of the burner according to the invention is notlimited to the tube geometry shown but can also be transferred torectangular channels or plate-shaped arrangements.

The hot water heater can also be used advantageously for heating warmair or another fluid to be heated.

We claim:
 1. A hot water heater,comprisinga first inlet for a liquidfuel; a plurality of inlets for air; a second inlet for a fluid to beheated; a first combustion stage traversed by a fuel-air mixture; asecond combustion stage traversed by the fuel-air mixture; a fluidchamber for said fluid from the second inlet; wherein said firstcombustion stage comprises a combustion chamber having an outer wall andincluding therein an evaporation chamber, said evaporation chamberhaving a catalyst layer at least partially surrounding an outer surfaceof said evaporation chamber; and wherein said fluid chamber at leastpartially surrounds said outer wall of said combustion chamber such thatexhaust gas from said evaporation chamber flows between said catalystlayer and said outer wall of said combustion chamber.
 2. A hot waterheater according to claim 1, wherein the combustion chamber of the firstcombustion stage is a catalytic cracking burner.
 3. A hot water heateraccording to claim 1, wherein the evaporation chamber is designed as acombustion chamber for starting the hot water heater, said evaporationchamber having a source of primary air and an ignition device and aheated inlet for liquid fuel.
 4. A hot water heater according to claim3, wherein the ignition device includes its own source of combustiongas.
 5. A hot water heater according to claim 1, wherein a fuel supplyto said evaporation chamber is thermally insulated.
 6. A hot waterheater according to claim 1, further comprising at least one of a nozzleand porous structure for at least one of atomization and evaporation ofthe liquid fuel.
 7. A hot water heater according to claim 6, whereinsaid nozzle is one of a piezocrystal, porous ceramic, and vorticizationnozzle.
 8. A hot water heater according to claim 1, further comprisingan opening provided for offgas recirculation from an outlet of the firstcombustion stage to the evaporation chamber.
 9. A hot water heateraccording to claim 1, further comprising a device provided in thecombustion chamber for guiding a gas stream.
 10. A hot water heateraccording to claim 1, wherein the combustion chamber is movable.
 11. Ahot water heater according to claim 10, wherein said combustion chamberis rotated.
 12. A hot water heater according to claim 1, wherein theouter surface of the evaporation chamber is a cylinder, said cylinderhaving at least a partial catalyst layer on an outer jacket surface.